The flame-retarded systems consisting of triphenylphosphate (TPP) and nano-poly(phenylsilsesquioxane) spheres (nano-PPSQ) for poly(methyl methacrylate) are reported. The flame-retarded effects were studied by a cone calorimeter test and thermal gravimetric analysis (TGA). The results show that when a part of TPP was substituted with nano-PPSQ, the values of the peak heat release rate of the samples are reduced. The burnt resides were analyzed by the scanning electron microscopy, Raman spectra, and X-ray photoelectron spectroscopy. The results provide evidences that nano-PPSQ can efficiently promote the formation of more dense and compact charred layers composed of varying amounts of graphite and amorphous carbon. The possible mechanisms were discussed. (C) 2011 Wiley Periodicals, Inc. Adv Polym Techn 30: 33-40, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.20205